Digital wireless communication systems are of increasing interest for all types of data and speech transmission. A frequently used method in particular for mobile cellular communications is code division multiple access (CDMA), e.g. according to the Universal Mobile Telephone System (UMTS). For CDMA the signal to be transmitted is typically spread to a multiple of its original bandwidth. The signal with spread bandwidth is less sensitive to interference and the spectral power density is reduced. Commonly, direct sequence CDMA (DS-CDMA) is used, where the signal is multiplied or correlated by a code sequence before modulation. The spread and correlated symbols are called chips. Using a plurality of code sequences being orthogonal to each other a plurality of communication connections can utilise the A same frequency band. Due to the orthogonality of the codes the transmitted signals can be decoded or decorrelated uniquely in the receiver. An advantageous group of code sequences are so-called pseudo-noise (PN) bit sequences which are typically used for direct sequence CDMA. CDMA and pseudo-noise correlation are known to those skilled in the art.
However, in a wireless communication system a transmitted signal may still propagate along different paths due to reflection or scattering. Therefore, the signal is received as a superposition of different signal components each propagated along one of the possible paths. As reflected signals will be differently delayed according to their paths compared to each other and compared to the direct signal, each of the signal components of one specific path is interfered by a plurality of other time- and phase-shifted signal components of other propagation paths. If there is no direct sight between the transmitter and the receiver the connection can still be provided by the reflected or scattered signal components, but their interference generally causes disadvantageous effects to the connection performance.
Also the CDMA radio channel is characterised by multipath propagation where a number of reflected or scattered radio rays arrive at the receiving end. Each of the rays, as seen by the receiver, is characterised by a distinct phasor and time-delay. The RAKE receiver is a commonly used structure to demodulate the DS-CDMA signal, suggested by the third Generation Partnership Project [3GPP] (see Technical Specification Group Radio Access Network; Physical layer—General description [3G TS 25.201 version 3.0.0] which is incorporated herewith by reference) as a low-complexity solution for fast time-to-market, and will be the receiver of choice for the first wave of CDMA handsets. In each finger of the RAKE receiver, the phase shift, and the propagation delay of the selected multipath have to be compensated for, a task called synchronisation. Any realisable receiver follows the concept of synchronised detection which is described in detail by Heinrich Meyr, Marc Moeneclaey and Stefan Fechtel, Digital Communication Receivers: Synchronization, Channel Estimation and Signal Processing, John Wiley and Sons, New York, 1998, which is incorporated herewith by reference, further referred to as reference [1] and for which a channel estimate or a sampled version thereof must be formed and subsequently used for detection as if it were the true known channel.
In the RAKE receiver the signal components being part of the received multipath signal, which is in fact a superposition of the signal components are summed to increase the signal height and to improve the signal-to-noise ratio (SNR). Before summing it is necessary to derotate and weight each signal component by the phasor.
Disadvantageously the quality of the estimate of the phasor itself is influenced by the multipath fading, resulting in a reduced efficiency of the signal component summation. Therefore, the signal height and the signal-to-noise ratio is worse than it could be, if the phasors would be known exactly. Resulting from this another disadvantageous effect is an increase of the bit error rate. In particular, if the delay between two signal propagation paths (signal paths) is small, i.e. shorter than one chip duration or in the range of one to two chip durations interference between the signal components of neighbouring paths is disadvantageously strong.